For telephone interfaces or for isolated DAAs, a controllable hook switch is necessary to close the telephone line loop current path in order to signal the telephone switch to answer or originate a call. Typically, a mechanical switch, an electro-mechanical relay, or an electronic switch provides this hook switch function. The minimum requirements of a hook switch are to block maximum normal line voltage (up to 300V), be able to pass up to 120 mA, and have an on-state resistance in the range of 0–100 ohms.
A hook switch may also provide dielectrical isolation between the switch and the control signal. Some examples of isolated hook switches are relays, mechanical switches, and optically isolated solid-state relays. However, dielectric isolation is not necessary for a hook switch if the source of the control signal is isolated. Such is the case in many electronic telephones or solid state DAAs. In these products, a controlling IC on the telephone line side generates signals that directly control the hook switch. The controlling IC responds to signals sent across a dielectrically isolated signaling means; such as, a transformer, optical isolator, or capacitor. In the current state of the art of DAAs, the hook switch commands are encoded along with audio and other signals that are sent across a shared isolated signal path.
Electronic hook switches are more commonly used in high volume telephone and DAA products because they offer lower cost than relays, consume less power, and are smaller. However, electronic hook switches are constructed from high voltage transistors and have difficulties being, driven from integrated circuits fabricated in low voltage processes. One difficulty is that the control current available on a DAA integrated circuit (IC) is very low. Another difficulty is that often at least one transistor is required to high voltage level shift control outside of the operating voltage range of the controlling circuit. Collectively, level shifting and cascading of transistors to get sufficient current gain typically require the use of at least 3 high voltage, bipolar transistors. Fewer transistors can be used if a more expensive high voltage enhancement metal oxide semiconductor (MOS) or a depletion MOS high voltage transistor is used. (See the prior art schematics shown in FIG. 1)
To explain in greater detail: The most common low cost electronic hook switch uses at least 3 high voltage bipolar transistors. At least this many transistors must be used because the beta of a high voltage transistor is low, e.g. in the range of 20–100, and the control current from a controlling IC may only be in the range of 0.1–10 uA. The control current is low because during on-hook idle less than a few microamperes of current may be pulled from the line in order to meet regulatory requirements. Although higher current may be available if the controlling switch can directly switch from the line, such a switch must be able to withstand the 300V peak line voltage. In practice. the controlling IC is usually fabricated in a low cost 5 volt or less process and hence cannot be directly powered from the line voltage.
One general method to overcome the low control current available from the line or the inability to power the hook controlling IC directly from the line is to send low voltage power across the isolation barrier. Conexant presently illustrates this method in their DAAs using a small high frequency transformer to send AC power across the isolation barrier to power the DAA IC. Although this method is quite effective, the power transformer is relatively expensive.
Another method illustrated in the optically isolated solid state switch is to send power across the isolation barrier in the form of light from a light emitting diode (LED) which falls on a stack of photodiodes in series which produces sufficient voltage and current to drive the gate of a high voltage MOS transistor which acts as a hook switch. Although this method is low power, it is also relatively expensive, requiring an LED, a photovoltaic stack, and 1–2 high voltage MOS transistors in a specialized optical package. This approach is demonstrated in FIG. 1a. 
Another method is to send power across isolation capacitors with an AC signal as disclosed by Silicon Laboratories (see, for example, U.S. Pat. No. 6,430,229). Although in capacitor isolated DAAs, controlling signals for hook and other functions are sent across capacitors, it is possible to send much more power across than is available from the line in idle mode. This is especially true where the isolation capacitors are in the range of 30 pF to 300 pF. With a 1 MHz AC signal with 5V peak-to-peak swing, up to 150 uA to 1.5 mA of current can be sent across these size capacitors. This level of current can be used to drive low-cost high-voltage bipolar transistors with fewer gain stages. This technique is less expensive than a transformer or optical solid-state switch but the high voltage isolation capacitors are moderately expensive. This approach is demonstrated in FIG. 1c. 
Another technique, illustrated in the linear opto-isolator DAA of U.S. Pat. No. 5,946,393, is to micro power the DAA IC controlling the hook switch with microamperes of current delivered through a current limiting resistor greater than 5 Meg ohm that is connected to the line. The IC provides an internal voltage clamp function to prevent over voltage damage. A resistor greater than 5 Meg Ohm is required to meet FCC part 68 on-hook DC resistance requirements for telephone devices. The internal micro power source is then used to drive the hook switch input with as little as 0.5 uA of current. See FIGS. 1b and 1d. 
In order to have adequate gain when using lowest cost high-voltage bipolar transistors, usually at least two transistors and sometimes more need to be cascaded in order to provide a current gain of several thousand in order to amplify the control current sufficiently to switch up to 120 mA of loop current. Plus, at least 1 bipolar transistor is necessary to provide a high voltage level shift function between the low voltage IC and the cascaded transistors since the voltage required to drive the base of the input transistor of the cascade is outside of the supply voltage range of the low voltage controlling IC. See FIGS. 1b, 1c and 1d. 
Besides the hook switch function, the low voltage, line powered ICs in telephone or DAA circuits require external voltage regulations protection from high voltage transients, and loop current limiting for European use. More high voltage transistors are typically required to provide these functions. Additionally, other high voltage components are necessary to provide low voltage signals for on-hook functions of RING detect, line status determination and Caller ID signal processing. For example, both RING detect circuitry and Caller ID line interfacing circuitry usually use high voltage capacitors of at least 250V to block the DC component of the line. See FIG. 1c. 